Google has revealed its self-driving cars crash "surprisingly often" -- but it's nearly always the human driving behind who is at fault.

Chris Urmson, who leads Google's automated cars program, wrote in a blog post that its "self-driving cars are being hit surprisingly often by other drivers who are distracted and not paying attention to the road".

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As an example, Google published a video of one of its autonomous Lexus cars on the roads in Mountain View, California, when it was forced to brake on a green light at an intersection due to traffic. After it stopped, a car "slammed" into it at 17mph having not braked at all.

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Google said the crash was an example of why it wants to develop self-driving cars in the first place; if cars were able to talk to each other, even if still nominally under the direction of a human driver, this and similar incidents would be avoided.

"As you can see from the video above, our braking was normal and natural, and the vehicle behind us had plenty of stopping distance ," Urmson claimed. "But it never decelerated. This certainly seems like the driver was distracted and not watching the road ahead. Thankfully, everyone in both vehicles was okay, except for a bit of minor whiplash, and a few scrapes on our bumper. The other vehicle wasn’t so lucky; its entire front bumper fell off."

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Google said the lesson was more than one of basic culpability, noting its stats essentially proved most accidents were not reported to the police, and current information about crashes-per-miles-driven was largely meaningless.

"Other drivers have hit us 14 times since the start of our project in 2009 (including 11 rear-enders), and not once has the self-driving car been the cause of the collision," he explained. "Instead, the clear theme is human error and inattention. We'll take all this as a signal that we’re starting to compare favourably with human drivers."

Clearly Google wants to promote its cars as the safe alternative to human drivers -- the entire future of the market for autonomous vehicles depends on it. But its point is still well made: most traffic collisions are small, avoidable and the result of human error. "The fight to end distracted driving starts with each of us -- at least until that day when you can summon a self-driving car and just kick back, relax, and enjoy the ride," Urmson wrote.